Effective index approximation based analytical modeling and two dimensional numerical investigation of surface and bulk sensitivity in optimized hybrid nanostructured plasmonic gratings with miniaturized footprints

Enhancement in surface and bulk sensitivity of surface plasmon resonance based grating sensor is significantly associated with efficient coupling of Transverse Magnetic (TM) polarized incident light with the sensor. This can be achieved by relevant usage of materials and by suitable remodeling of grating structural geometry. Different configurations have already been explored for improvement in bulk sensitivity but the enhancement of surface sensitivity is major consideration for biological and chemical sensing applications. The investigations reported in literature are mostly based on numerical techniques that are explored for sensitivity analysis of grating structures. There is a need for development of compact physics based analytical model for calculation of sensitivities of grating structures. In this context, we propose two nanostructured plasmonic gratings made of gold and ZnS with miniaturized footprint in x–z dimensions. The gratings are obtained by nested optimization of design parameters using finite difference time domain method (FDTD). Mathematical validation is done for verifying the size of footprint of gratings. Calculations indicate that any dielectric material like Si₃N₄, MoO₃, GaN and ZnS with refractive index varying in range of 2.2–2.6 can be used in design. This makes our design more versatile in terms of materials to be selected for grating structures. In depth analysis of surface sensitivity is done for the first time. The grating structure 1 depicted surface sensitivity of 53.425 nm/RIU and bulk sensitivity of 300 nm/RIU at visible wavelength of 781.25 nm. Grating structure 2 had bulk sensitivity of 117.55 nm/RIU at resonance dip 1 of near infrared wavelength of 1108.64 nm. A novel technique is proposed for enhancing sensitivity by incorporating rectangular trench in dielectric layer of grating structures. Spatial field pattern analysis shows enhancement in coupling of TM polarized light to surface plasmons at the interface thus resulting in increase in sensitivity. New parameter not used before is introduced to measure this surface sensitivity in enhanced novel structure. Surface sensitivity of grating structure 1 is improved to 93.11 nm/RIU while bulk sensitivity gets reduced to 288.13 nm/RIU. High surface sensitivity of 116.25 nm/RIU is obtained in grating structure 2 at dip 1, while both high surface and bulk sensitivity of 463.45 nm/RIU is obtained at far infrared wavelength of 3067.48 nm. This is the highest value of surface sensitivity reported so far. As a second major contribution, we made an effort to obtain a compact physics based analytical model using effective index approximation method for critical evaluation of bulk and surface sensitivity in designed grating structures. The proposed analytical model corroborates the trend obtained from simulations and thus can be safely used for sensitivity analysis in plasmonic grating structures.